Planar optical wave guides typically comprise a buffer or so-called undercladding layer deposited on a silicon substrate. This buffer layer may be silicon oxide, undoped or doped with one or more of boron, phosphorus and germania. Deposited on the buffer layer is a core layer, also typically of doped silicon oxide. There may be a stress compensating oxide layer deposited on the underside of the substrate, to counteract any tendency of the oxides on top to cause thermal or other distortions. Optical signals are contained within the core layer by a lower refractive index arranged for the surrounding buffer and a cladding layer deposited on the core layer.
In such an arrangement, the cladding layer is generally deposited by a chemical vapour deposition process, typically plasma enhanced chemical vapour deposition. The material is deposited in a form that requires consolidation into a coherent vitreous composition and this is effected by an annealing process. A particular problem with current annealing processes is the excessive time, typically several hours, required for their completion to provide a dense uniform material. It was previously considered necessary to raise the temperature to the annealing temperature at a very slow rate to avoid the risk of cracking and consequent degradation of the deposited cladding layer. This in turn impairs the optical properties of the wave guide core. It is believed that this cracking of the cladding layer results from voids or imperfections that are left from the evolution of bonded volatile materials, particularly hydrogen, that are present in the deposited material prior to the annealing process. The need for long annealing times has made these devices relatively costly to manufacture and has thus restricted their general use.